The present invention relates to the field of compression of ultrashort optical pulses.
Since the invention of mode-locked lasers, considerable effort has been directed towards the generation of ultrashort optical pulses. Novel techniques for broad-band dispersion control now enable self-mode-locked Ti:Sapphire lasers to directly produce 6.5 femtosecond pulses. Compression of pulses down to sub-5 fs is achievable by treating the outputs of such lasers with novel spectral broadening techniques in external pulse compressors. Efficient pulse compression generally requires characterization of the pulses. Grating-pair or prism-pair compressors are commonly used to compensate mainly for second order dispersion, while a combination of these allows for simultaneous compensation for the second and the third orders, as described, for instance in an article by R. L. Fork, C. H. B. Cruz, P. C. Becker and C. V. Shank, entitled xe2x80x9cCompression of optical pulses to six femtoseconds by using cubic phase compensationxe2x80x9d published in Optics Letters, Vol. 12, p. 483 (1987). More recently, chirped dielectric mirrors, tailored to produce negative group velocity dispersion over a wide spectrum, have been described by R. Szipxc3x6cs, K. Ferencz, C. H. Spielmann, and F. Krausz, in the article entitled xe2x80x9cChirped multilayer coating for broadband dispersion control in femtosecond lasersxe2x80x9d which appeared in Optics Letters, Vol. 19, p. 201 (1994). Such chirped mirrors have been used to compress pulses down to durations of sub-5 fs, as described in the article entitled xe2x80x9cCompression of high-energy laser pulses below 5 fs.xe2x80x9d by M. Nisoli, S. De Silvestri, O. Svelto, R. Szipxc3x6cs, K. Ferencz, Ch. Spielmann, S. Sartania, and F. Krausz, published in Optics Letters, Vol. 22, p. 522 (1997).
However, in cases where the pulses are uncharacterized, or when the spectral phase cannot be approximated by the leading few terms of the corresponding Taylor expansion, these techniques for pulse compression cannot be used efficiently, since the spectral transfer function needed to form the desired output pulse cannot be calculated. A specific spectral transfer function corresponds to a specific complex input pulse spectrum. Consequently, compression of arbitrary uncharacterized pulses down to the minimum time-bandwidth product cannot be accomplished by these prior art methods since the relative phases between the spectral components of the input pulses are not known.
Furthermore, practical considerations limit the use of such techniques to situations where the pulse source is substantially constant in time. Actual laser sources undergo slow variations in time, therefore severely limiting the usefulness of these techniques for the compression of ultrafast pulses. As the speed of optical communication increases, the disadvantages of currently available pulse compression technologies become more and more felt. There therefore exists a critical need for a faster, more efficient, versatile pulse compressor, capable of handling arbitrary optical pulses with durations of the order of femtoseconds.
The present invention seeks to provide an improved pulse compressor, which overcomes the disadvantages and drawbacks of existing ultrashort pulse compressors, especially with respect to their application to pulses which are completely uncharacterized or partially characterized, or which originate from a laser source whose output varies with time. An improved femtosecond pulse compressor according to the present invention, has been first described by applicants in an article entitled xe2x80x9cAdaptive ultrashort pulse compression and shapingxe2x80x9d published in Optics Communications, Vol. 138, pp. 345-348 (June 1997), and experimental results obtained with such a pulse compressor have been reported by applicants in an article entitled xe2x80x9cAdaptive femtosecond pulse compressionxe2x80x9d, published in Optics Letters, Vol. p. 22, pp. 1793-1795 (December 1997). These articles, as well as the disclosures of all publications mentioned in this section and in the other sections of the specification, and the disclosures of all documents cited in any of those publications, are hereby incorporated by reference.
The femtosecond pulse compressor according to the present invention uses an adaptive technique. The use of adaptive control for pulse shaping was first suggested by R. S. Judson and H. Rabitz, in their article entitled xe2x80x9cTeaching lasers to control moleculesxe2x80x9d, published in Physical Review Letters, Vol. 68, p. 1500 (1992), and later by A. M. Weiner in his extensive review article entitled xe2x80x9cFemtosecond optical pulse shaping and processingxe2x80x9d, published in Progress in Quantum Electronics, Vol.19, pp. 161-237 (1995). In those articles, however, the authors suggest using the adaptive procedure for pulse shaping only, such pulse shaping being used, for instance, in optimizing the yield of laser induced chemical reactions. No suggestion was made anywhere in these articles to use an adaptive technique for pulse compression, for use, for instance, in optical communication networks.
There is thus provided in accordance with a preferred embodiment of the present invention, an adaptive pulse compressor, especially for use with ultrashort pulses, wherein the input pulses are modified in an iterative fashion, according to a feedback signal obtained from measurement of the output pulses. The feedback signal is derived from any measured property of the output pulse that increases with the intensity of the pulse as it is compressed. Any non-linear property may be used as the phenomenon from which to derive this feedback signal, such as the generation of a second harmonic signal from the pulse by means of a non-linear crystal. This feedback signal is used with a suitable optimization algorithm to control the spectral components of the incoming pulses, so as to maximize the intensity of the output pulse, and hence to provide maximum compression. The control is performed by means of a programmable spatial light modulator, such that almost arbitrary phase functions can be realized to accomplish efficient compression The adaptive femtosecond pulse compressor according to the present invention, thus removes one of the main disadvantages associated with prior art pulse compressors, namely, the need for characterization of the uncompressed pulses. Thus, the use of the adaptive technique in this invention, together with the ability to form almost arbitrary spectral phase and/or amplitude filtering, allows the efficient and versatile compression of arbitrary pulses.
Additional important benefits arise from the use of adaptive pulse compression. For example, the adaptive scheme can be used not only to compress pulses, but also to correct for slow variations of the laser source. Another benefit arises from the fact that once the optimal filter has been calculated as a result of the compression process, this filter essentially provides a measurement of the spectral phase of every component of the original pulse. This data, together with the power spectrum, allows for full characterization of the pulses.
There is further provided in accordance with another preferred embodiment of the present invention, an adaptive femtosecond pulse compressor, including an adaptive optical pulse compressor for compressing input pulses into output pulses of shorter duration, the input pulses being modified by an iterative procedure according to a feedback signal obtained from a measurement of the output pulses.
In accordance with yet another preferred embodiment of the present invention, there is provided an adaptive optical pulse compressor as described above, and wherein the input pulses are either uncharacterized or partially characterized, or where the input pulses are time varying.
In accordance with a further preferred embodiment of the present invention, there is also provided an adaptive optical pulse compressor as described above, and wherein the iterative procedure uses an optimization algorithm, which may preferably be a simulated annealing optimization procedure.
In accordance with still another preferred embodiment of the present invention, there is provided an adaptive optical pulse compressor as described above, and wherein the iterative procedure is an optimization algorithm with constraints as to the range of dispersion coefficients to be used in the Taylor expansion expressing the input pulse characteristics.
There is further provided in accordance with yet another preferred embodiment of the present invention, an adaptive optical pulse compressor as described above, and wherein the input pulses have durations of the order of femtoseconds.
There is further provided in accordance with still another preferred embodiment of the present invention, an adaptive optical pulse compressor for compressing input pulses into output pulses of shorter duration, including a programmable pulse shaper, optical apparatus for providing a feedback signal dependent on the intensity of the output pulses, and a computing system operative to control the programmable pulse shaper according to the feedback signal.
There is further provided in accordance with yet another preferred embodiment of the present invention, an adaptive optical pulse compressor as described above, and wherein the programmable pulse shaper includes an input dispersive device for spatially separating frequency components of the input pulse, a programmable spatial light modulator, a focusing element for focusing the spatially separated frequency components onto the programmable spatial light modulator, a focusing element for collimating the spatially separated frequency components after transmission through the programmable spatial light modulator, and an output dispersive device for recombining the spatially separated frequency components outputted from the programmable spatial light modulator.
There is provided in accordance with still a further preferred embodiment of the present invention, an adaptive optical pulse compressor as described above, and wherein at least one of the input and output dispersive devices is a holographic grating, a ruled grating or a prism.
There is even further provided in accordance with a preferred embodiment of the present invention, an adaptive optical pulse compressor as described above, and wherein at least one of the focusing elements is a lens or a mirror, or wherein at least one of the dispersive devices is also operative as a focusing element.
There is also provided in accordance with a further preferred embodiment of the present invention, an adaptive optical pulse compressor as described above, and wherein the programmable spatial light modulator is operative to control at least the phase of the individual spatially separated frequency components of the input pulse.
In accordance with yet another preferred embodiment of the present invention, there is provided an adaptive optical pulse compressor as described above, and wherein the feedback signal is a non-linear measurement of the output pulse.
In accordance with a further preferred embodiment of the present invention, there is also provided an adaptive optical pulse compressor as described above, and wherein the optical apparatus for providing a feedback signal dependent on the intensity of the output pulses comprises a harmonic generator and a photo-detector.
In accordance with still another preferred embodiment of the present invention, there is provided an adaptive optical pulse compressor as described above, and wherein the harmonic generator is a second harmonic generator, which may be a non-linear crystal.
There is further provided in accordance with yet another preferred embodiment of the present invention, an adaptive optical pulse compressor as described above, and wherein the optical apparatus for providing a feedback signal dependent on the intensity of the output pulses comprises apparatus for providing a multi-photon fluorescence output signal and a photo-detector.
There is further provided in accordance with still another preferred embodiment of the present invention, an adaptive optical pulse compressor as described above, and wherein the multi-photon fluorescence output signal is a two-photon fluorescence output signal.
There is provided in accordance with still a further preferred embodiment of the present invention, an adaptive optical pulse compressor as described above, operative to correct for distortion in the output pulses of a laser or a laser amplifier chain.
There is also provided in accordance with a further preferred embodiment of the present invention, an adaptive optical pulse compressor as described above, operative to provide dispersion compensation in an optical data communication channel.